Yes, PLANETARIUMS is a Scrabble word!

Letter values in Scrabble US: P 3, L 1, A 1, N 1, E 1, T 1, A 1, R 1, I 1, U 1, M 3, S 1.

Is it a valid word?

PLANETARIUMS is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a building housing an instrument for projecting the positions of the planets onto a domed ceiling
an optical device for projecting images of celestial bodies and other astronomical phenomena onto the inner surface of a hemispherical dome
an apparatus or model for representing the solar systems